The Yellow Diamonds are the most common colored diamonds so they are less costly than others. The demand for yellow diamonds is very high and only the Pink Diamonds can match their demand. The demand for a certain color rises drastically if influential individuals obtain the diamond. This was seen when Jennifer Lopez was proposed to with a pink diamond ring and Paris Hilton was proposed to with a yellow diamond ring.
The rarest diamonds are sold in auction houses for tens of thousands of dollars and this can even go up to millions. Diamond enthusiasts don't hesitate to part with their money for such precious stones.
Basically, pure red diamonds do not exist or rather have not been found so far. The available ones are fancy purplish-red or brownish-red. The red color in combination with other high-quality 3Cs makes them priceless. The two famous red diamonds ever sold are the Moussaieff Red and the Hancock Red Diamond. These diamonds are among the most famous diamonds ever mined. The current value could be extraordinarily high.